Granular sodium tripolyphosphate is commonly prepared by first reacting phosphoric acid and a base such as sodium hydroxide or soda ash to provide sodium orthophosphate (orthophosphate). This mixture is then heated to at least about 300.degree. C. and usually between about 300.degree. C. to about 600.degree. C. to form sodium tripolyphosphate (STPP). In many instances a solution of orthophosphate is fed into a spray dryer or rotary kiln dryer to convert the orthophosphate to STPP. Whether STPP is prepared from dried orthophosphate or orthophosphate solutions there is produced STPP in particle size ranges which provide various bulk densities depending on specific conditions. However, common to all processes is the production of undesirable, small sized particles or fractions which are then separated from the desired STPP. Such under-sized particles are generally referred to as "fines". Fines are generally defined as particles less than 100 USS mesh. Such fines are undesirable in a commercial product because these particles cause dust which can irritate nasal membranes, etc., if not separated from the more desirable granular fractions.
Because of the problems caused by fines they are separated from the useful fraction and many attempts have been made to convert such fines into useful products. One such example is found in U.S. Pat. No. 3,160,472 to Metcalf et al whereby the fines are calcined in the presence of at least about 10% of certain blends of alkali metal orthophosphate whereby an appreciable portion of the fines is agglomerated. Another attempt to utilize the fines is found in German Patent DE 3509661 to Klee et al wherein the fines are compacted alone or in admixture with at least one other finely divided washing powder component with a quantity of water required to provide 100% hydration. This material is compacted between rollers, then crushed again to obtain the proper size material followed by removing the under sized and over sized particles.
All of the processes of the prior art create inefficiencies in recycling STPP and do not totally eliminate the production of fines. Large scale production of STPP is common because it has been the material of choice in various detergent applications as a builder in the commonly used built detergent. The tonnage needed to supply this demand causes the production of large amounts of fines. In the production of granular product, excess production of fines leads to a lose of product unless they can be either used or efficiently reprocessed or avoided in the production process. There is needed an efficient method whereby STPP is produced from orthophosphate in particle sizes which largely avoids or greatly reduces the amount of fines produced.